(1) Field of the Invention
This invention relates to a device for detecting photoelectrons in air which will be emitted from solid surfaces by a work function of photon irradiation energy using different photoelectric work functions of respective solids in place of using function of kinetic energy, especially to a device for suppressing background noise as adds false counting rate to true counting rate measured on a detection sample, which will be produced when photoelectrons are emitted by scattered reflection from the detection sample.
(2) Description of the Prior Art
An open counter for photoelectron detection was disclosed in Japanese Patent Open Publication No. 60-262005 (application No. 59-118,818 filed 1984), in contrast with photo electron detection in an ultra-high vacuum. Such a counter was also disclosed by the inventor in the paper, Japanese Journal of Applied Physics Vol. 24 (1985), Supplement 24-4. pp. 284-288.
The photoelectron detection chamber 1 of the device partially shown in FIG. 1A comprises a grid G6 for quenching gas discharge, a grid G7 for suppressing and neutralizing positive ions and an anode A5 using a loop-shaped tungsten filament, wherein the grid G6, G7 and anode A5 are respectively supplied with +100 V, +80 V and +3.5 kV with respect to the chamber 1 grounded as a cathode. The electrons were emitted from the subject upon irradiation by photons and accelerated by two grids to become attached to air or O.sub.2 molecules to form negative ions in the atmosphere of the detection chamber. Near the anode A5 electrons were detached from negative ions and caused a gas discharge which was induced by high positive electric fields applied to the anode (+3.5 kV). By detecting a reduction in the high voltage on the anode due to an initial discharge, quenching was carried out by supplying a positive square pulse (+300 V in amplitude and of about 3 msec in width Te) to the quenching grid G6. Some of the positive ions produced around the anode could pass through the quenching grid G6 during a discharge followed by quenching. To neutralize these positive ions, -30 V was supplied to the suppressor grid G7. Such a series of procedures prevented successive and continuous discharges, and enabled electrons in an atmosphere to be counted without either self-quenching or internal quenching. After positive ions were neutralized, the grids G6 and G7 are turned to the initial voltages referred to FIG. 1B.
Secondary discharge incurred by positive ions was avoided by repeating the above procedure for stably counting the rate of photoelectrons in air.
When a ray source spectroscope in FIG. 1A which can change the wave-length of supplied irradiation energy from a lower value (longer wavelength) to a higher value, a certain amount of energy causes photoelectron emission due to the photoelectric effect. The counting rate (Hz) of photoelectrons per second and ray irradiation energy (hv) have the following relationship:
(Hz).sup.n .alpha.hv, wherein n=0.4.about.1 usually, n=1/2 for metal.
Photoelectron emission energy is given by a work function and the value of the work function is different for different kinds of substances. In the case of an oxide layer on silicon, the work function of the oxide is larger than that of silicon so that presence of the oxide layer reduces the amount of photoelectrons emitted from that emitted from silicon. When the photoelectrons are given an irradiation energy which is larger than the work function of silicon, the counting rate of photoelectrons N is given as follows:
log N=N.sub.0 -T/2.3.lambda., wherein T is the thickness of the layer: N.sub.0 is the counting rate at a thickness of zero: .lambda. is the mean free path in the oxide layer. Generally, other substances have equations similar to this. A value of N of 350.about.1 Hz was confirmed in response to thicknesses from 0.about.140 A.degree.. In the above noted paper, compensation was also disclosed for atmospheric pressure, temperature and humidity changes. However, there is no suggestion in the abovenoted paper as to the problem of scattered ray irradiation causing a false increase in the photoelectron counting rate as background noise. It is impossible to obtain an exact counting rate of photoelectrons using suitable compensation equations unless the background noise of scattered ray irradiation was suppressed.